Flow regulating device



April 11, 1950 w. G. FINLAYSON FLOW REGULATING mavrcm Filed May 3, 1946INVENTOR. Mum: [,2 HM arse/v Patented Apr. 11, 1950 FLOW REGULATINGDEVICE William G. Finlayson, Detroit, Mich., assignor to ,VickersIncorporated, Detroit, Mich., a corporation of Michigan Application May3, 1946, Serial No. 667,173

15 Claims.

This invention relates to power transmissions, particularly to those ofthe type comprising two or more fluid pressure energy translatingdevices, one of which may function as a pump and another as a fluidmotor.

The invention is particularly concerned with flow regulating valves ofthe compensating by pass type for use in hydraulic transmissionscontaining a fluid pump, a reversible fluid motor and throttling meansfor regulating the speed of the motor. This type of valve has been usedin the past to maintain a constantly regulated flow of fluid through thethrottle, doing this by being responsive to the pressure ahead of andbeyond the throttle for maintaining a uniformly constant pressure dropacross the throttle. However, where the speed of the motor was to beregulated in both directions and a throttling efl'ect was to be producedby the partial opening in both directions of a directional controlvalve, the problem of maintaining a constant pressure drop across boththrottles was solved by using a separate compensating valve for eachthrottle.

It is an object of this invention to provide in a hydraulic systemhaving a fluid pump, a reversible fluid motor, and a directional controlvalve which may be opened partially to provide a throttling means foreach end of the motor, means combined with a single compensating bypassvalve for causing the compensating valve to maintain a uniformlyconstant pressure drop across both throttles.

It is a further object to provide for a hydraulic system, as abovementioned, a single compensating valve of the by-pass type responsive topressure ahead of both throttles and a pressure-actuated shuttle valvein combination therewith to cause the compensating valve to beresponsive to the pressure drop beyond the throttle through which fluidis being directed to the motor.

It is also an object of this invention to provide in combination with a,compensating valve of the by'-pass type a simply constructed,economically manufactured shuttle valve for the purpose above statedand, in addition, a pressure responsive, pilot-control valve for causingthe compensating valve to relieve the pump of excessive pressure fluid.

Further objects and advantages of the present invent on will be apparentfrom the following descriptlon, reference being had to the accompanyingdrawing wherein a preferred form of the present invention is clearlyshown.

In the drawing the single figure shown is a diagrammatic view of asimple hydraulic system incorporating a sectional view of a preferredform of the present invention.

Referring now to the single figure, there is shown a pump I B which maybe driven by a prime mover, not shown, which is connected to a tank l2by means of a suction conduit l4 and which is connected by means of apump discharge conduit l6 to the pressure port l8 of a four-waydirectional control valve 20. A pair of motor ports 22 and 24 ofdirectional control valve 20 are connected to opposite ends of abalanced-type cylinder motor 26 by means of fluid conduits 28 and 30,respectively, the motor 26 having a piston 32 reciprocably mountedtherein'to which is connected a piston rod 34 extending from each end ofthe motor. A tank port 36 of valve 20 is connected to tank i2 by meansof a conduit 38 and a conduit 40'.

A branch conduit 42 connected to pump delivery conduit i6 connects thepump Hi to a pressure port 44 of a flow control valve 46. A branchconduit 48 connected to motor conduit 30 connects the latter conduit toa cylinder port 50 of the valve 46, and a branch conduit 52 connected tomotor conduit 28 connects the latter conduit to another cylinder port 54of valve 46.

The valve 20 is provided with a centrallylocated longitudinal bore 56which is directly connected to the pressure port I! and the motor ports22 and 24 and which is connected to the tank port 36 by means of apassage 58 having two arms connecting the passage 58 to opposite ends ofthe bore 56. Reciprocably mounted in the bore 58 is a spool 60 providedwith a centrally-located land 62 and lands 64 and 66 spaced apart to theright and left. respectively, of central land 62. Spool 60 is providedwith a centrally-located longitudinal passage 68 which is directlyconnected to a plurality of ports 18 to the left oi. land 66, aplurality of ports to the right of land 64, and a plurality of ports82in central land 62. The lands 62 and 64 cooperate to control theadmission of fluid from the pressure port I8 to either of the motorports 22 or 24, and the plurality of ports 82 in cooperation with thepassage 62 and the plurality of ports 18 and 80 control the return fiowof fluid from motor 26 flowing in motor conduits 28 or 30 so as toproperly direct said return flow to tank port 36 through the medium ofpassage 58. The spool 60 may be shifted completely in either directionso as to open the pressure port It completely to motor port 22 or 24, orit may be variably partially shifted in either direction so as topartially open motor port 22 or 24 to pressure port It for providing athrottling effect for the flow of fluid to motor 28 through motorconduit 28 or 30. Thus, the effect of the partial opening of thepressure port and the motor port produces an effect exactly identical tothat of-having a variable throttle in each of the motor conduits 28 and30.

Referring now to flow regulating valve 48, there is shown a compensatingby-pass valve 84 reciprocably mounted therein which is comprised of apiston 88 having its under-surface area exposed to pressure within aninlet chamber 88 which is connected to the pressure port 44 of valve 48by means of a passage 90 and its upper surface exposed to pressurewithin a control chamber 92. A spring 94 of predetermined resistancebiases piston 88 so that a valve 98 formed at the lower end thereofcloses a seat 98 which is connected to an exhaust chamber I00. Exhaustchamber I is connected to tank I2 by means of a passage I02 which isdirectly connected to an exhaust port I04, the latter being connected totank I2 by means of conduit 40.

A shuttle valve I08 is adapted to connect either the motor conduit 28 orthe motor conduit 30 to the control chamber 92 of valve 84. Shuttlevalve I08 is reciprocably mounted in a bore I08 and is provided with endlands H0 and II 2. Cylinder port 50 is connected to bore I08 by means ofa passage II4 which intersects a groove II8 of bore I08 controlled byland I I0. Cylinder port 50 is also connected to the end surface of landII2 by means of a passage H8 and a passage I which although on aslightly different plane that of bore I08 intersects bore I 08, passageII4 not only intersecting groove II8 but also intersecting passage II8.Cylinder port 54 is connected to bore I08 by means of a passage I22which intersects a groove I24 of bore I08. Cylinder port 54 is connectedto the end surface of land IIO by means of a passage I28 whichintersects a passage I28; the latter passage being connected to theextreme end of bore I08 by means of two passages I29 and I3I. PassagesH8 and I20 lie in a different plane from that of passages I28 and I28 sothat the two sets of passages do not connect with each other.

The shuttle valve I08 is adapted to be actuated upwardly when thecontrol valve 20 directs pressure fluid to the motor 28 through motorconduit 30 and downwardly when pressure fluid is directed to the motorthrough motor conduit 28. The shuttle valve I08 will block communicationbetween the control chamber 92 and the end of the motor from which fluidis flowing to tank I2 while at the same time expose the upper surface ofpiston 88 within chamber 92 to the pressure in the motor conduit throughwhich pressure fluid is flowing to the motor. Bore I08 is connected to avertical passage I30, the latter of which is connected to controlchamber 92 of compensating valve 84 by a. passage I32, by means of apassage I34 which connects a groove I38 of bore I08 with verticalpassage I30. A restriction I 38 is formed in passage I34 for a purposehereinafter mentioned.

Control chamber 92 of valve 84 is connected to a small poppet reliefvalve I40 by means of a passage I42 and a passage I44 intersectingpassage I42. When a predetermined pressure has arisen in control chamber92 as determined by the resistance of a spring I48; relief valve I40will open passage I44 to exhaust chamber I00 of compensating valve 84 bymeans of a passage I48.

In operation, with the pump I0 running and spool 80 of control valve 20shifted, as shown. 50

I0 through conduit I8 and enter the pressure port I8 of valve 20.Pressure fluid will also flow by means of conduit 42 connected to pumpdischarge conduit I8 to the pressure port 44 of flow-regulating valve 48where, by means of passage 90, it flows into the inlet chamber 88 ofcompensating valve 84, exposing the under-surface of piston 88 to thepressure within said chamber. Pressure fluid entering pressure port I8is directed by control valve 20 to conduit 28 and thence flows to theupper end of motor 28, shifting piston 32 downwardly. Due to the factthat pressure port I8 and motor port 22 have only been opened partiallyby land 82, a drop in pressure takes place across port I8. This drop inpressure across pressure port I8 is present in control chamber 92 to actupon the upper surface of piston 88 by means of conduit 52, cylinderport 54', passage I22, groove I24, bore I08, groove I38, passage I34,passage I30 and passage I32. The drop in pressure across port I8 presentin conduit 28 is present in control chamber 92 because shuttle valve I08has been actuated by pressure fluid in conduit 28 to open cylinder port54 of valve 48 to the control chamber 92 of valve 84. Pressure fluid inconduit 28, being delivered to the upper end of motor 28,,is alsodelivered to cylinder port 54 by means of conduit 52 and by means ofpassage I22 to vertical passage I28 where it flows to the upper end ofbore I08 to act on the extreme surface of land I I0 by means of passageI28 and passages I28 and I3I. The pressure force necessary to shiftshuttle valve I08 being much less than that needed to shift piston 32 ofmotor 28 shuttle valve I08 will be actuated completely before the motormay operate. -Fluid displaced from the lower end of bore I08 by the landI I2 is conducted to the motor port 24 of directional control valve 20by means of passages I20, 8, and H4 and port 50 of shuttle valve 48, andconduits 48 and 30, and thence to the tank I2 by means of the transverseports 82, passage 88, and transverse ports of valve spools 80, valvebore 58, passage 58, port 38, and conduits 38 and 4'0.

Due to the fact that the under-surface of piston 88 is equal to theupper surface of said surface and that the pressure ahead of thethrottle is acting on the under-surface of the piston and the pressurebeyond the throttle is acting on the upper surface of the piston, theresistance of spring 94 will determine the pressure drop across thethrottle I 8. If this pressure drop tends to become greater or less thanthe resistance of the spring 94, this increase and decrease of pressuredrop will also be present in inlet chamber 88 and control chamber 92 toshift piston 88 upwardly or downwardly thereby shifting the valve 98 tomore fully open or close the seat 98 to exhaust chamber I 00. Only anamount of fluid will be bypassed sufilcient to maintain the flow acrossthe throttle I8 constant.

If a, rapid-traverse movement of the motor 28 is desired, the spool 80of control valve 20 may be shifted completely to the left from theposition indicated on the drawing so as to open fully the pressure portI8 to the motor port 22. In such a case, due to the fact that piston 88is only responsive to a pressure drop across an existing throttle whichno longer exists because spool 80 has been shifted completely, thecompensating valve 84 will be closed and permit the rapid movement ofpiston 32 of motor 28.

OJIOUJO? It the motor 26 is to be operated in the opposite direction anda regulated feed movement is desired, spool 60 is shifted rightwardlybut not completely so as to partially open pressure port I8 to motorport 24. Pressure fluid from pump II will now be delivered by pumpdischarge conduit I6 to the pressure port I8 of control valve 20 andalso by means of branch conduit 42 to the pressure port 44 offlow-regulating valve 46. Due to the fact that the pressure port I8 hasbeen only opened partially to motor port 24, a throttle is created atport I8, and the pressure ahead of said throttle will be present ininlet chamber 88 to act on the under-surface of piston 86. Pressurefluid flowing across the throttle created by land 62 at port I8 will bedelivered by means of conduit 30 to the lower end of motor 26, shiftingpiston 32 upwardly and at the same time by means of branch conduit 48connected to motor conduit 30 to the motor port 50 of valve 46 fromwhence by means of passage II4 it is delivered to passage H8 and bymeans of passage I20 to bore I08 where it may act upon the extreme endsurface of land 2, shifting shuttle valve I06 upwardly. Due to the factthat the pressure force needed to shift shuttle valve I06 is much lessthan that needed to shift piston 32 of motor 26, shuttle valve I06 willmove and shift completely to the extreme position before motor 26 beginsto operate in the opposite direction. Fluid displaced from the upper endof bore I08 by the land H is conducted to the motor port 22 ofdirectional control valve 20 by means of passages I29 and I3I, passagesI28, I26, and I22 and port 54 of shuttle valve 46, and conduits 52 and28, and thence to the tank I2 by means of transverse ports 82, passage68, and transverse ports 18 of valve spool 60, valve bore 56, passage58, port 36, and conduits 38 and 40. I

When shuttle valve I06 has shifted completely upwardly, land IIO willhave opened groove II6 to passage I I4, and pressure fluid in saidpassage may enter bore I08 from whence it may flow to the controlchamber 92 of valve 84 by means of passage I34, restriction I38, passageI30 and passage I32. Pressure fluid in bore I08 is prevented fromflowing into passage I22 by means of land II2 of valve I06 blockingcommunication therefrom. When shuttle valve I06 has moved completelyupwardly, piston 32 of motor 26 will begin to shift, and compensatingvalve 84 will maintain the flow across the throttle created at port I8constant. Pressure ahead of pressure port I8 will be present in inletchamber 88, and pressure past the throttle at port I8 will be present incontrol chamber 92. If the pressure drop across the throttle tends to begreater or less than the resistance of spring 94, piston 88 will shift,opening or closing seat 98 more fully to exhaust chamber I00, and asufficient amount of fluid will be by-passed to tank I2 to maintain theflow across throttle I8 constant.

If a rapid traverse movement of motor 26 is desired in the upperdirection, spool 60 is shifted completely rightwardly so as to openfully pressure port I8 to motor port 24, and the full flow of pressurefluid from pump I0 will be delivered to the lower end of motor 26 whilecompensating valve 84 remains closed.

The block 46 is so constructed that all its connection ports come out atthe bottom surface thus lending it to use with a gasket mounted valveblock. Thus it may be bolted against any flat surface which has matingconduit openings formed to register with the ports of the valve block.Preferably the mounting surface may be formed on the four-way valve 20in which case the branch passages 48 and 62 are unnecessary.

It should be noted thatany conventional type of four-way directionalcontrol valve may be used providing that the directional control valvemay be opened partially in both directions so as to produce'a throttlingeffect in the motor conduits 28 and 30. In addition, variable throttlescould be incorporated in motor conduits 28 and 30, the directionalcontrol valve shifted completely in both directions, and the throttlesin such motor conduits could be operated manually or by pressureresponsive means. In its preferred form, however, the invention is usedwith the type of four-way directional control valve shown for the reasonthat the operator may at any time determine the degree of opening inorder to have a variable feed movement of the motor and the operator mayat any time produce a rapid-traverse of th motor 26 in either direction.

It should also be noted that the shuttle valve I 06 is simply designedand economically manufactured and that, when used in combination withthe compensating by-pass valve 84, it is a simple and an eflicient meansof producing feed movement in both directions of a reversible fluidmotor with the use of only one compensating valve.

Whenever the pressure in motor conduit 28 or motor conduit 30 reaches apredetermined maximum, this pressure is also present in control chamber92 and, by means of passage I42 and passage I44, may act upon pilotcontrol valve I40 to shift the same against the resistance offered byspring I 46 and opening control chamber 92 to exhaust chamber I00 bymeans of passage I48. Due to the fact that pressure fluid in controlchamber 92 may bleed through the opening created by the shifting ofpilot control valve I40 faster than pressure fluid may enter the controlchamber 92 through the restriction I38, a sudden unbalancing of pressureforces takes place within inlet chamber 88 and control chamber 92,causing the increase of pressure in inlet chamber 88 to shift the piston86 upwardly and resulting in the valve 96 opening seat 98 completely toexhaust chamber I 00. All exce sive pressure fluid from pump I0 is thencompletely by-passed by means of compensating valve 84 to the tank I2.

Whenever the pressure in motor conduit 28 or 30 drops below theresistance offered by spring I46, the spring I46 will shift the valveI40 to close communication between the control chamber 92 and exhaustchamber I00, thus again balancing the pressure forces in inlet chamber88, control chamber 92 and permitting spring 94 to shift the piston 86,and the valve 96 will close seat 98.

- trol valve I40 used in combination with the com- While the form ofembodiment of the invention 7 as herein disclosed constitutes apreferred form, it is to be understood that other forms might be whichfollow.

What is claimed is as follows:

1. In a hydraulic power transmission contain ing a fluid pump forming asource of pressure fluid, a reversible fluid motor and a directionalcontrol valve for selectively directing fluid to and from the motor foreither direction of operation thereof, said control valve having a rangeof travel in each direction for providing a variable throttle for eachend of the motor, the combination of a flow regulating valve and apressure actuated shuttle valve for maintaining a uniformly constantpressure drop across the throttle through which fluid is being directedby the control valve to the motor.

2. In a hydraulic power transmission containing a fluid pump forming asource of pressure fluid, a reversible fluid motor and a directionalcontrol valve for selectively directing fluid to and from the motor foreither direction of operation thereof, said control valve having a rangeof travel in each direction for providing a variable. throttle for eachend of the motor, the combination of a flow regulating valve containinga compensating valve having opposed piston areas one area of which iscontinuously exposed to pressure ahead of the throttles and a pressureactuated shuttle valve for exposing the remaining compensating valvepiston area to pressure beyond a throttle through which pressure fluidis being d rected to the motor thereby maintaining a unii'ormly constantpressure drop across said throttle.

3. In a hydraulic power transmission containing a fluid pump forming asource of pressure fluid, a reversible fluid motor and a directionalcontrol valve for selectively directing fluid to and from the motor foreither direction of operation thereof, said control valve having a rangeof travel in each direction for providing a variable throttle for eachend of the motor, the combination of a flow regulating valve having abypass and a compensating valve for controlling the by-pass, saidcompensating valve having opposed piston areas one area of which iscontinuously exposed to pressure ahead of the throttles, and a pressureactuated shuttle valve for exposing the remaining piston area topressure beyond a throttle through which pressure fluid is beingdirected to the motor thereby maintaining a uniformly constant pressuredrop across said throttle.

4. In a hydraulic power transmission containing a fluid pump forminga'source of pressure fluid, a reversible fluid motor and a directionalcontrol valve for selectively directing fluid to and from the motor foreither direction of operation thereof, said control valve having a rangeof travel in each direction for providing a variable throttle for eachend of the motor, the combination of a flow regulating valve having abypass and a compensating valve for controlling the by-pass, saidcompensating valve having opposed piston areas one area of which iscontinuously exposed to pressure ahead of the throttles, and a shuttlevalve connected to both ends of the motor beyond the throttles and tothe remaining piston area of the compensating valve, said shuttle valvebeing responsive to pressure fluid flowing to the motor to expose theremaining piston area to pressure beyond a throttle through whichpressure fluid is flowing to the motor whereby the compensating valvemaintains 'a uniformly constant pressure drop acrosssaidthrottle.

5. In a hydraulic power transmission containing a fluid pump, areversible fluid motor. a directional control valve and separate fluidconduits connecting the control valve to the pump, to both ends of themotor and to an exhaust source. said control valve being of the type forselectively directing fluid to and from the motor for either directionof operation thereof and having a range of travel in each direction forproviding a variable throttle for each motor conduit, the combination ofa flow regulating valve containing a by-pass, a compensating valve forcontrolling the by-pass and means resiliently biasing thecompensatingvalve to close the bypass, said compensating valve havingopposed piston areas one area of which is continuously exposed topressure ahead of the throttles, and a shuttle valve connected beyondthe throttles to the motor conduits and to the remaining piston area ofthe compensating valve, said shuttle valve being responsive to pressurefluid in a motor conduit through which fluid is flowing to the motor forconnecting said conduit to the remaining piston area of the compensatingvalve whereby the compensating valve maintains a uniformly constantpressure drop across the throttle through which pressure fluid isflowing to the motor.

6. In a hydraulic power transmission containing a fluid pump forming asource of pressure fluid, a reversible fluid motor and a directionalcontrol valve for selectively directing fluid to and from the motor foreither direction of operation thereof, said control valve having a rangeof travel in each direction for providing a variable throttle for eachend of the motor, the combination of a compensating by-pass valve and apressure actuated shutt e valve for maintaining a uniformly constantpressure drop across the throttle through which fluid is being directedby the control valve to the motor and means res onsive to predeterminedincreases in pressure for causing the compensating by-pass valve toperform the function of a relief valve.

'7. In a hydraulic power transmission containing a fluid pump forming asource of pressure fluid, a reversible fluid motor and a directionalcontrol valve for selectively directing fluid to and from the motor foreither direction of operation thereof, said control valve having a rangeof travel in each direction for providing a variable throttle for eachend of the motor, the combination of a compensating by-pass valve and apressure actuated shuttle valve for maintaining a uniformly constantpressure drop across the throttle through which fluid is being directedby the control valve to the motor and a pilot control valve responsiveto predetermined increases in pressure for causing the compensatingby-pass valve to perform the function of a relief valve.

8. In a hydraulic power transmission containing a fluid pump forming asource of pressure fluid, a reversible fluid motor and a directionalcontrol valve for selectively directing fluid to and from the motor foreither direction of operation thereof, said control valve having a rangeof travel in each direction for providing a variable throttle for eachend of the motor, the combination of a flow regulating valve and apressure actuated shuttle valve for maintaining a uniformly constantpressure drop across the throttle through which fluid is being directedby the control valve to the motor, said flow regulatingvalve containinga by-pass, a compensating valve for controlling the by-pass havingopposed piston areas one of which is connected to the pump ahead of thethrottles, and resilient means of predetermined resistance biasing thecompensating valve to close the by-pass, and the shuttle valve beingconnected to the remaining piston area of the compensating valve and tothe directional control valve beyond the throttles, said shuttle valvebeing responsive to pressure past the throttle through which pressurefluid is flowing to the motor for exposing the remaining piston area ofthe compensating valve to pressure beyond said throttle.

9. In a hydraulic power transmission containing a fluid pump forming asource of pressure fluid, a reversible fluid motor and a directionalcontrol valve for selectively directing fluid to and from the motor foreither direction of operation thereof, said control valve having a rangeof travel in each direction for providing a variable throttle for eachend of the motor, the combination of a flow regulating valve and apressure actuated shuttle valve for maintaining a uniformly constantpressure drop across the throttle through which fluid is being directedby the control valve to the motor, said flow regulating valve containinga by-pass, a compensating valve for controlling the by-pass havingpposed piston areas one of which is connected to the pump ahead of thethrottles, and resilient means of predetermined resistance biasing thecompensating valve to close the by-pass, and the shuttle valve beingconnected to the remaining piston area of the compensating valve and tothe directional control valve beyond the throttles,

said shuttle valve being responsive to pressure past the throttlethrough which pressure fluid is flowing to the motor for exposing theremaining piston area of the compensating valve to pressure beyond saidthrottle, and means responsive to predetermined increases in pressurefor causing the compensating by-pass valve to perform the function of arelief valve.

10. In a hydraulic power transmission containing a fluid pump and areversible fluid motor the combination of a directional control valvefor reversing the direction of operation of the motor, means in thecontrol valve for creating a throttle for each end of the motor uponshifting the control valve a predetermined range in each direction, aflow regulating valve containing a bypass, a compensating valve forcontrolling the by-pass, and resilient means of predetermined resistancebiasing the valve to close the by-pass, said compensating valve havingopposed piston surface areas one offwhich is continuously exposed .topressure ahead of both throttles, and a hydraulically actuated shuttlevalve connected to both ends of the motor beyond the throttles and tothe remaining piston surface of the compensating valve for exposing thelatter surface to pressure beyond the throttle through which pressurefluid is flowing to the motor whereby the comresistance biasing thevalve to close the by-pass, said compensating valve having opposedpiston surface areas one of which is continuously exposed to pressureahead of both throttles, and a hydraulically actuated shuttle valveconnected to both ends of the motor beyond the throttles and to theremaining piston surface of the com pensating valve for exposing thelatter surface to pressure beyond the throttle through which pressurefluid is flowing to the motor whereby the compensating valve isresponsive to the pressure drop across the throttle through which fluidis flowing to the motor.

12. In a hydraulic power transmission having a fluid pump forming asource of pressure fluid, a reversible fluid motor, and means forming apair of throttles one for each end of the motor the combination of acompensating by-pass valve having opposed piston surface areas one ofwhich is continuously exposed to pressure ahead of the throttles and ahydraulically actuated shuttle valve responsive to the direction of flowthrough said throttles for exposing the opposite piston surface area topressure beyond a throttle through which fluid is being directed to themotor.

13. In a hydraulic power transmission having a fluid pump forming asource of pressure fluid, a reversible fluid motor, and means forming apair of throttles one for each end of the motor the combination of acompensating by-pass valve having opposed piston surface area one ofwhich is continuously exposed to pressure ahead of the throttles, ahydraulically actuated shuttle valve responsive to the direction of flowthrough said throttles for exposing the opposite piston surface area topressure beyond a throttle through which fluid is being directed to themotor whereby the compensating valve maintains a uniformly constantpressure drop across both throttles and means responsive to apredetermined pressure for also causing the compensating valve toperform the function of a relief valve.

14 In a hydraulic power transmission having a fluid pump forming asource of pressure fluid and-a reversible fluid motor the combination ofa directional control valve for reversing the directional operation ofthe motor, means in said control valve for forming a pair of throttles,one for each end of the motor, during a predetermined range of shiftingthe control in each direction, a compensating by-pass valve havingopposed piston surface areas one of which is continuously exposed topressure ahead of the throttles and a hydraulically actuated shuttlevalve connected to the remaining piston surface area responsive to theposition of the control valve for exposing the remaining surface area topressure beyond a throttle through which fluid is being directed to themotor.

15. In a hydraulic power transmission having a fluid pump forming asource of pressure fluid and a reversible fluid motor the combination ofa directional control valve for reversing the directional operation ofthe motor, means in said control valve for forming a pair of throttles,one for each end of the motor, during a predetermined range of shiftingthe control in each direction, a compensating by-pass valve havingopposed piston surface areas one of which is continuously exposed topressure ahead of the throttles, a hydraulically actuated shuttle valveconnected to the remaining piston surface area responsive to theposition of the control valve for exposing the remaining surface area topressure beyond a throttle through which fluid is being directed to themotor whereby the compensating valve maintains a uniformly constantpressure drop across the latter throttle and means responsive to apredetermined pressure for causing the compensating valve to perform thefunction of a relief valve.

' WIHJAM G. FINLAYSON.

REFERENCES CITED The following references are of record in the file ofthis 'patent:

UNITED STATES PATENTS Number Name Date 2,102,865 Vickers Dec. 21, 19372,157.70! Keel May 9, 1939

